The 37-year-old Lester, who had surgery in March to remove a parathyroid gland, was one of 11 players to be put on the injured list before the season opener after four had tested positive for COVID-19.
"Hopefully he's beyond all this and he's going to come back and start his career with the Nationals today," manager Davey Martinez told reporters Friday. "He's just another veteran starter that [Mike] Rizzo and I thought could help us win many games.
"I know the guys are excited about it and he's excited about it."
Lester, a free-agent pickup from the Chicago Cubs during the offseason, had been preparing at the team's alternate training site in Fredericksburg, Virginia.
His operation was March 5 for hyperparathyroidism, which can affect the amount of calcium levels in the bloodstream and lead to someone tiring easily.
Lester, a five-time All-Star with three World Series rings, went 3-3 in 2020 with a 5.16 ERA in 12 starts for the Cubs.
"His veteran presence is a big thing," Martinez said. "We feel like he has a lot left in the tank. We're looking forward to getting him into that five-man rotation and go from there. He's one of those guys I respect very much."